1. Field of the Invention
The present invention relates to a multi-hop wireless communication system including a plurality of wireless terminals that performs multi-hop communication with a wireless base station.
2. Description of the Related Art
In a multi-hop wireless communication system, a wireless signal generated by one wireless terminal is normally transmitted to a wireless base station through another wireless terminal in a multi-hop fashion although the wireless signal may also be transmitted directly to the wireless base station. A sensor network as a typical example application of the multi-hop wireless communication system requires that the cost is reduced using small batteries since a number of wireless terminals need to be installed. The sensor network also requires that power consumption is reduced to allow sensing data to be repeatedly reported to the base station over a long period of time. From the viewpoint of maintenance, the sensor network also requires low power consumption to eliminate the need to replace batteries frequently over a long period.
Japanese Patent Application Publication No. 2005-354634 discloses a technique which achieves low power consumption by allowing each wireless terminal that performs multi-hop wireless communication to activate its own wireless transmitter/receiver only when a wireless signal addressed to an ID thereof is received.
Japanese Patent Application Publication No. 2007-116408 discloses a technique which achieves low power consumption by synchronizing a multi-hop wireless communication network to allow each wireless module to be turned off except when the wireless module is waiting to receive signals or is receiving or transmitting signals.
However, when such an intermittent operation method as the above conventional technologies is employed, a number of wireless terminals in the network may collectively shift from an inactive state to an active state at the same time to start transmitting wireless signals, as shown in FIG. 1. This causes problems in that wireless signals of adjacent wireless terminals collide with each other and idle waiting time occurs, thereby hindering efficient communication. For example, in the configuration described in Japanese Patent Application Publication No. 2005-354634 (see a description of paragraph [0039] and FIG. 4), a multi-hop route to which wireless terminals 34 and 36 belong is activated at a certain time during a period for which a multi-hop route to which wireless terminals 33 and 35 belong is activated. As a result, there is a possibility that wireless signals of the wireless terminals 33 and 34 will collide, thereby hindering efficient communication. In addition, in the configuration described in the Japanese Patent Application Publication No. 2007-116408, it is not possible to avoid collision of wireless signals of wireless terminals in different multi-hop routes although low power consumption is achieved.